The present invention relates generally to frequency converters and more particularly to thyristor frequency converters including protection means responsive to a commutation failure of the thyristor.
Frequency conversion systems generally employ silicon controlled rectifiers, hereinafter referred to as SCR's. In these frequency converters, commutation of the SCR is provided during each cycle by energy storage in capacitor or inductor. If for any reason the SCR should fail to commutate, the usual consequence is a short circuit across the low frequency power lines. To prevent damage of the SCR and other related circuitry, fast-acting fuses and circuit breakers are commonly employed to open the short circuit.
The requirement for high reliability and maintenance-free frequency converters has forced engineers to select energy storage elements with large safety margins to assure that commutation occurs reliably even during transients in load impedance. This requirement for large commutation safety margins has tended to limit the use of SCR frequency converters in such applications as high frequency induction heating where the load impedance is erratic or undefined, reliability without maintenance is essential, or where economic factors prevent use of large safety margins in selecting commutating energy storage elements.
U.S. Pat. No. 3,821,630 discloses a protection circuit for an SCR converter in which the instantaneos voltage across the SCR is sensed and compared with a threshold voltage to inhibit the current flow through the SCR when the instantaneous voltage across the SCR is less than the threshold voltage. The converter includes a transistor chopper circuit which is responsive to gating signals to periodically interrupt the current flow through the SCR. During the inhibit period the chopper circuit is also inhibited to allow the SCR to restore to its blocking state.
However, the transistor chopper circuit should be of a relatively heavy duty type to allow sufficient magnitude of current to flow through the SCR, and if the frequency converter is designed to operate as an induction heating system the chopper circuit would have to carry a large value of current, which would affect the reliability and economy of the system.